1. Field of Invention
This invention pertains to light emitting diode (LED) lamp-lighting circuits that can light without fail and without flickering, In addition, the invention relates to LED lamps that have an LED as a light source, as well as to LED lamps and conversion socket devices used for lighting LED lamps that have said lamp-lighting circuits.
2. Description of Related Art
In recent years, for reasons of long-life, low power consumption, and the like, people have begun using lamps that use LEDs (light emitting diodes) (hereinafter “LED lamps”) as the source of light for illumination.
For example, Japanese Unexamined Patent Application Publication 2006-344919 describes an LED lamp-lighting circuit that is used for illumination. FIG. 5 of this document shows an example of a lamp-lighting circuit that lights multiple LEDs.
FIG. 13 shows the lamp-lighting circuit of FIG. 5 of Japanese Unexamined Patent Application Publication 2006-344919. As shown in that figure, multiple LEDs are connected in a series/parallel configuration and the AC voltage that is stepped down by the voltage transformer T2 is rectified in the current rectifying circuit DB, smoothed in the capacitor C3 and then applied to the LEDs, which are connected in the series/parallel configuration described above via the current-limiting resistors R3, lighting them.
The LEDs are driven using direct current (DC) and when lighting LEDs using an AC power supply, the AC voltage must be rectified and converted into DC, as stated in the Japanese Unexamined Patent Application Publication 2006-344919 described above.
FIG. 14 shows an example of a lamp-lighting circuit that uses an LED driver. As shown in FIG. 14, the lamp-lighting circuit is made up of a current rectifying circuit 101, which has the current-rectifying diodes D1, D2, D3 and D4, and it rectifies AC, converting it into DC. It also has an LED drive circuit that has a smoothing circuit 102 that includes the capacitor C1 and the LED driver 103, which is equipped, among other things, with a DC-DC converter that drives the LED.
It is also noted that the LED driver 103 is equipped with a current-fixing function that keeps the current supply to the LED uniform. For example, if 12 V AC voltage was being supplied to the LED drive circuit described above, the LED would light up.
Generally, for example, the power supply voltage to the LED drive circuits described above would step down 100 V of commercial use AC voltage to a supply of 12 V.
In order to step down 100 V of commercial use AC voltage to 12 V, for example, requires the use of a transformer, but in recent years, in order to make devices more compact, AC-AC converters, so-called electronic transformers, have been gaining in use in place of the more conventional coil-wrapped, iron-core transformers (copper-iron transformers).
Electronic transformers either do not use the copper-iron transformer described above or they use a compact copper-iron transformer and step the AC voltage up or down. When using a transformer, the frequency is set higher than that of the commercial-use current, allowing for more compact transformers.
A variety of methods is known for electronic transformers and they can be divided into the internally-excited type of electronic transformers, like the Hartley oscillators, blocking oscillators or similar internally-excited oscillators and the externally-excited electronic transformers.
FIGS. 15(a) & 15(b) show a schematic example of internally-excited and externally-excited electronic transformer configurations, respectively.
As in the figures, commercial AC voltage with a frequency of 50/60 Hz and a voltage of 100 V is converted into DC voltage by a rectifying circuit 111 and then smoothed in a smoothing circuit 112, which has a capacitor CC, before being applied to an internally-excited oscillation circuit 113.
The internally-excited oscillation circuit 113 of FIG. 15(a) is made up of tertiary windings (feedback windings), a transformer Tr1, which has the tertiary windings LL3, a switching element Q1, a resistor R1 and a diode D1.
Voltage is applied to the primary windings LL1 of the transformer Tr1 by switching the switching element Q1 and, along with the generation of voltage in the secondary windings LL2 of the transformer Tr1, voltage is also generated in the tertiary windings LL3. The voltage that is generated in the tertiary windings LL3 returns to the switching element Q1 as positive feedback. This makes the circuit composed of the switching circuit Q1 and the transformer Tr1 to perform the internally-excited oscillation operation, which causes the switching operation of the switching element Q1 to repeat. What this does is to generate an AC voltage of 12 V with a frequency of 20 kHz to 100 kHz in the secondary windings LL2 of the transformer Tr1.
FIG. 15(b) is a sample schematic configuration of an externally-excited electronic transformer. As shown in that figure, the rectification circuit 111 converts the commercial voltage of 100 V and a frequency of 50/60 Hz to a DC voltage, which is smoothed in the smoothing circuit 112, which has the capacitor CC, before being applied to the externally-excited oscillation circuit 114.
The externally-excited oscillation circuit 114 is made up of the transformer Tr2, the switching element Q2 and an IC or other generator circuit OSC. The switching element Q2 is driven by the oscillation circuit OSC and the frequency of said oscillation circuit repeats the switching operation. What is does is to generate 12 V of AC voltage with a frequency of 20 kHz to 100 kHz at the secondary windings LL2 of the transformer Tr2.
However, the illumination equipment for a conventional retail store uses halogen lamps or a similar type of lamp. When using halogen lamps that are powered using 12 V AC for illumination, power supply devices containing the electronic transformers described above might be installed up behind the ceiling with power supplied to the lamps from a socket mounted in the surface of the ceiling.
In recent years, there has been increasing demand for LED lamps as substitutes for the halogen lamps described above because they use less power and it is desired that it be made possible to use the halogen lamp power supply equipment without any modification to power LED lamps instead of halogen lamps.
When lighting a lamp using voltage that is lower than commercial AC voltage, a transformer is used on the power supply circuit to step the voltage and because of miniaturization in recent years, it has become common to use electronic transformers.
Electronic transformers include internally-excited and externally-excited types, but when using the internally-excited type of electronic transformers as a power supply for LED lamps, there has been a problem of the LED failing to light up or for lighting to be discontinuous.
This is due to the fact that, if a load current that is greater than a certain degree does not flow, the internally-excited transformer cannot generate stable oscillations and it is thought that in the LED, no current can flow in the low voltage range where the DC-DC converter of the drive circuit operates.
FIG. 16 shows the voltage-current characteristics of an LED and a halogen lamp. As shown in that diagram, a current runs through the halogen lamp that corresponds to the voltage being applied, but no current flows at the LED's low voltage range, and no current will flow until a voltage that is above a certain point is applied.
For this reason, when using an internally-excited electronic transformer as the power supply for an LED lamp, the load current that flows is insufficient, so that the oscillations are not stable.
Because the above problem does not occur unless one uses an internally-excited electronic transformer in the power supply circuit, an externally-excited electronic transformer or similar solution may be used when installing a new power supply circuit to light the LED lamp, which makes it possible to use a power supply circuit that can light the LED lamp in a stable manner. However, it is also possible that LED lamps might be installed instead of halogen lamps in existing illumination equipment that lights halogen lamps, in which case the LED lamp would not be lit stably if an internally-excited electronic transformer were used.
The user does not always know what type of circuit is being used in the power supply circuit of the illumination equipment so that it is possible that an LED lamp could be installed, for example, where the existing equipment is for halogen lamps in which an internally-excited electronic transformer is being used, and in this case, the LED lamp would not go on normally and could give the mistaken impression that the LED lamp was broken.